Laser glass is produced by melting in a platinum vessel because it is required to have high uniformity. Thus the laser glass inevitably contains platinum inclusions. With a recent increase of a laser output, it has been revealed that platinum colloids (platinum inclusions) of such size and number that have been accepted cause damage on the glass due to laser, and formation of such platinum colloids is becoming a serious problem in production of laser glass.
Particularly in laser glass to be used as an amplifier for nuclear fusion, since an output of 8 to 10 J/cm.sup.2 is sought, platinum inclusions are responsible for thermal damage; they are vital defects. These platinum inclusions are 5 to 30 microns in size.
In connection with the mechanisms of formation of platinum inclusions, it is reported by R. J. Ginter, J. Non-Cryst. Solids, Vol. 6, p. 294 (1971) that a vapor of platinum oxide evaporated from a platinum vessel condenses on low temperature areas, for example, on the upper portion of a shaft of a rotary stirrer, separates therefrom and drops, and is reduced in a molten glass, resulting in the formation of platinum inclusions.
Based on this theory, the following methods have been employed to prevent the formation of platinum inclusions. That is, one of the methods is to remove a platinum oxide vapor while preventing oxidation of platinum by flowing a reducing gas in a zone made of platinum, as described in the above J. Non-Cryst. Solids. Another physical method is such that the above shaft is provided with a flange to prevent condensed platinum from dropping into the molten glass.
These methods, however, fail to completely remove platinum inclusions. The reason for this is that the mechanism of formation of platinum inclusions also includes the phenomenon of deposition of platinum eluted from the vessel into the molten glass as a result of super saturation. All the above conventional methods are to control the molten glass from outside. Thus, the methods do not have any relation with the formation of platinum colloids in the molten glass; therefore, they cannot prevent the formation of platinum colloids or rather the reducing gas accelerates the formation of platinum colloids.